Diseases in plants cause considerable crop loss from year to year resulting both in economic deprivation to farmers and additionally in many parts of the world to shortfalls in the nutritional provision for local populations. The widespread use of fungicides has provided considerable security against plant pathogen attack. However, despite $1 billion worth of expenditure on fungicides, worldwide crop losses amounted to approximately 10% of crop value in 1981 (James, 1981; Seed Sci. & Technol. 9: 679-685).
The severity of the destructive process of disease depends on the aggressiveness of the pathogen and the response of the host. One aim of most plant breeding programs is to increase the resistance of host plants to disease. Typically, different races of pathogens interact with different varieties of the same crop species differentially, and many sources of host resistance only protect against specific pathogen races. Furthermore, some pathogen races show early signs of disease symptoms, but cause little damage to the crop. Jones and Clifford (1983; Cereal Diseases, John Wiley) report that virulent forms of the pathogen are expected to emerge in the pathogen population in response to the introduction of resistance into host cultivars and that it is therefore necessary to monitor pathogen populations. In addition, there are several documented cases of the evolution of fungal strains which are resistant to particular fungicides. As early as 1981, Fletcher and Wolfe (1981; Proc. 1981 Brit. Crop Prot. Conf.) contended that 24% of the powdery mildew populations from spring barley, and 53% from winter barley showed considerable variation in response to the fungicide triadimenol and that the distribution of these populations varied between varieties with the most susceptible variety also giving the highest incidence of less susceptible types. Similar variation in the sensitivity of fungi to fungicides has been documented for wheat mildew (also to triadimenol), Botrytis (to benomyl), Pyrenophora (to organomercury), Pseudocercosporella (to MBC-type fungicides) and Mycosphaerella fijiensis to triazoles to mention just a few (Jones and Clifford; Cereal Diseases, John Wiley, 1983).
Cereal species are grown world-wide and represent a major fraction of world food production. Although yield loss is caused by many pathogens, the necrotizing pathogens Septoria and Pseudocercosporella are particularly important in the major cereal growing areas of Europe and North America (Jones and Clifford; Cereal Diseases, John Wiley, 1983). In particular, the differential symptomology caused by different isolates and species of these fungi make the accurate predictive determination of potential disease loss difficult. Consequently, the availability of improved diagnostic techniques for the rapid and accurate identification of specific pathogens will be of considerable use to field pathologists.
Four Septoria species parasitize the small grain species. Septoria tritici is the causative agent of leaf blotch and is virulent on wheat but also parasitizes triticale and rye. It typically causes leaf necrosis. Septoria nodorum is the causative agent of glume blotch and is parasitic on wheat, triticale, rye and barley and although mainly restricted to glumes is also found on leaf blades and sheaths. Septoria avenae is parasitic on oats, wheat and triticale and Septoria passerinii is restricted to barley. Septoria diseases occur in all wheat growing areas at economically important levels. Different Septoria diseases frequently occur concurrently within fields and on individual plants, where the disease symptoms may be collectively referred to as the "Septoria complex". Typically, the most commonly found species are S. tritici and S. nodorum. According to Wiese (1977; Compendium of Wheat Diseases, Amer. Phytopath. Soc. pages 42-45), the Septoria complex presently destroys nearly 2% of the world's wheat annually, the yield loss being mainly the result of impaired grain filling. Fungicide treatments can save up to 20% in cases of severe Septoria infection, but it is often difficult to distinguish between the different Septoria species at the onset of infection and this makes the decision whether or not to invest in fungicide use difficult because different cultivars display differing degrees of resistance to the various Septoria species.
The eyespot disease of cereals is caused by the fungus Pseudocercosporella herpotrichoides and is restricted to the basal culm of the plant. Wheat, rye, oats and other grasses are susceptible to the eyespot disease which occurs in cool, moist climates and is prevalent in Europe, North and South America, Africa and Australia. Wheat is the most susceptible cereal species, but isolates have been identified which are also virulent on other cereals. The R-strain of the fungus, for example, has also been isolated from rye and grows more slowly on wheat than the W-strain which has been isolated from wheat. Although eyespot may kill tillers or plants outright, it more usually causes lodging and/or results in a reduction in kernel size and number. Yield losses associated with eyespot are of even greater magnitude than those associated with Septoria tritici and Septoria nodorum. Typical control measures for eyespot include treatment wit h growth regulators to strengthen intemodes, and fungicide treatment. However, the differing susceptibility of cultivars to different strains of the fungus render the predictive efficacy of fungicide treatments difficult.
Sigatoka leaf spot of banana occurs in two forms each of which is caused by a different fungus. The economically important Black Sigatoka is caused by Mycosphaerella fijiensis, whereas the less economically significant Yellow Sigatoka is caused by Mycosphaerella musicola (Johanson and Jeger, 1993; Mycol. Res. 97: 670-674). Black Sigatoka is the major problem in banana causing severe losses of 30% and more. Due to occurrence of fungicide resistance in Mycosphaerella fijiensis, usage of fungicide should best be limited to prevent the further occurrence of resistance. Consequently, the availability of diagnostic tools will provide an important means of identifying the appropriate circumstances in which to utilize fungicides without unnecessarily risking the development of further resistance.
Thus, there is a real need for the development of technology which will allow the identification of specific races of pathogen fungi early in the infection process. By identifying the specific race of a pathogen before disease symptoms become evident in the crop stand, the agriculturist can assess the likely effects of further development of the pathogen in the crop variety in which it has been identified and can choose an appropriate fungicide if such application is deemed necessary.